Alkali metals constitute group I of the S-block of the periodic table. The alkali metals are characterised by the presence of one s-electron in the valence shell of their atoms.(b) Atomic and ionic size : The alkali metal atoms have the largest sizes in a particular period in the periodic table.(c) Ionisation enthalpy: The ionisation enthalpies of the alkali metals are considerably low. A. Physical Properties:(i) Physical state : Allotments are silvery white, soft and light metals.(ii) Density : Their densities are quite low and increase from lithium to caesium. However, potas¬sium is lighter than sodium.(iii) Melting and Boiling points : They are low because of the weak bonding in the crystal lattices of the metals.(iv) On account of their low ionisation energies, these metal atom have great tendency to lose ns1 electron and form positive ions.